Light hydrocarbon ratios are of value in assessing maturity and alteration, particularly by evaporative fractionation and biodegradation,
also the source rock type of the unaltered oil.
LIMITATIONS DUE TO ALTERATION
Two widely occurring alteration processes, often concurrent, limit the direct interpretation of LHC ratios in terms of maturity
and kerogen/source rock type: evaporative fractionation and biodegradation. Allowance must be made for these effects.
In an alternative view, "aberrant" LHC ratios are diagnostic of the occurrence and degree of alteration due to these processes.
Details of the modifications attributable to evaporative fractionation are presented in Section 10, those due to biodegradation in Section 11.
Brief discussions are given here.
The gasoline range composition of petroleums is extensively modified by evaporative fractionation processes, i.e. by either light
end addition or light end depletion. Degree is best determined by reservoir fluid analysis which provides an evaluation
of the parameters E3 and E7, enrichment or depletion in the P3 and P7 pseudo-component ranges, as defined
in the Glossary above. The derivation of E7 was originally detailed in Thompson, (2010), Organic
Geochemistry, 41, pp. 370-385.
Statistics on the frequency of gasoline addition or depletion are available for only a single basin, that of Western Canada, which,
although extremely large (400,000 sq km), is overall gas-prone. This contrasts the condition often encountered in basin distal regions
(remote from the gas-generative center) where unaltered oils may occur. Figure 42 provides
statistics on E7 in 193 reservoir fluids (oils) from western Canada. Of these, 52% exhibit values greater than unity, evidencing the widespread
addition of C6-C7 light ends, while 34% exhibit values of E7 less than unity, evidencing depletion and P6-P7 loss.
Figure 43 is a histogram of the distribution of E7 in 29 gas-condensate
reservoir fluids from Alberta, but only those in which the P6+ fraction is sufficient to allow the determination of E7.
Gas-condensates are generally (72%) light end enriched, contributing to elevated values of E7 in oils.
The principal characteristic of evaporative fractionation observed in the gasoline range is enrichment in aromatic hydrocarbons.
Figure 44 illustrates the gas-chromatographic n-alkane profiles of a suite of oils from the
US Gulf of Mexico shelf Tertiary exhibiting progressive degrees of biodegradation, indexed by pristane/n-C18 and n-C10/n-C10*. The parameter
n-C10/n-C10* is a measure of the original quantity of n-decane, calculated from the slope of the n-C15+ fraction, while n-C10 represents the actual
quantity present. n-C10/n-C10* is covariant with pristane/n-C18 where pristane, highly branched, is resistant to degradation while the linear n-alkane
is highly susceptible. Figure 44 also demonstrates the depletion of the C6-C7 fraction with biodegradation, combined with evaporative loss in the
least biodegraded case.
The principal characteristics of biodegradation observed in the gasoline range are enrichments in branched hydrocarbons and naphthenes.
LHC COMPOSITIONAL RATIOS IN OILS
The standard LHC inter-compound ratios defined above (from Thompson, 1983, Geochimica et Cosmochimica Acta, 47, p 312)
are presented for oils in numerous basins in Table 7 . Subsequent
illustrations are derived from these data and others added later from the author's collection.
Figure 45 illustrates the distributions of values of H and I in Cretaceous and Permian oils
from the western United States (Table 7 and Thompson, 1983). The Cretaceous oils are derived from Type II kerogens, particularly the Mancos Shale,
the Permian oils from Type IIS kerogens in the Phosphoria Formation. Both suites are distributed close to the Generation Curve defined in sediment
extracts, interpreted as indicating limited alteration. The difference in diagram regions occupied by the two suites is usually diagnostic of source
type. Type II oils of elevated maturity exhibiting H - I values in the diagram region of Type IIS oils are rare, but it is not proven that Type IIS oils are
"supermature" compared to average Type II oils, as was originally suspected (Thompson, 1983). Differing ranges of paraffinicity parameters in the
two classes occur principally because of differences in generative mechanisms. Additionally, both biodegradation and evaporative fractionation
decrease the values of H and I, so that Type IIS oils may plot in the Type II region and both may lie below the curve and approach the origin.
Figure 46 illustrates the values of H and I in a suite of oils from Cook Inlet, Alaska,
derived from a Jurassic lacustrine source (Thompson, 1994). They are unaltered, and exhibit characteristically low H and I values for oils of 35 to
38 API gravity.
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